fix(session): use datetime UTC and tighten timestamp logging

src/va_agent/session.py

@@ -6,7 +6,7 @@ import asyncio
 import logging
 import time
 import uuid
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from typing import TYPE_CHECKING, Any, override
 
 from google.adk.errors.already_exists_error import AlreadyExistsError
@@ -112,8 +112,13 @@ class FirestoreSessionService(BaseSessionService):
         if hasattr(value, "timestamp"):
             try:
                 return float(value.timestamp())
-            except Exception:  # pragma: no cover - defensive fallback
-                logger.debug("Failed to convert timestamp %r", value)
+            except (
+                TypeError,
+                ValueError,
+                OSError,
+                OverflowError,
+            ) as exc:  # pragma: no cover
+                logger.debug("Failed to convert timestamp %r: %s", value, exc)
         return default
 
     # ------------------------------------------------------------------
@@ -185,7 +190,7 @@ class FirestoreSessionService(BaseSessionService):
                 )
             )
 
-        now = datetime.now(timezone.utc)
+        now = datetime.now(UTC)
         write_coros.append(
             self._session_ref(app_name, user_id, session_id).set(
                 {
@@ -384,7 +389,7 @@ class FirestoreSessionService(BaseSessionService):
         # Persist state deltas
         session_ref = self._session_ref(app_name, user_id, session_id)
 
-        last_update_dt = datetime.fromtimestamp(event.timestamp, timezone.utc)
+        last_update_dt = datetime.fromtimestamp(event.timestamp, UTC)
 
         if event.actions and event.actions.state_delta:
             state_deltas = _session_util.extract_state_delta(event.actions.state_delta)